This invention relates to a key-in sound-generating circuit for use in a multifunction telephone set capable of storing data according to the key-in operation.
A tone dial telephone set is usually provided with a keyboard containing dial keys DK and function keys FK, as is shown in FIG. 1. A dual tone multifrequency (DTMF) is assigned to each of these dial keys (DK). Each of the dial tone multifrequencies is formed by mixing a low-frequency signal at a low group frequency of 697 Hz, 720 Hz, 852 Hz, or 941 Hz, and a high-frequency signal at a high group frequency of 1209 Hz, 1336 Hz, 1447 Hz, or 1633 Hz. Function keys FK include re-dial key RK for calling a number dialed just before, and memory keys MK for calling numbers previously registered. This type of multifunction telephone set comprises a memory, a DTMF signal-generating circuit (tone dialer), and a microcomputer consisting of one chip, for example. The memory stores the key data input by operating various types of keys. The microcomputer stores into the memory key data input by operating keys on the keyboard. The microcomputer drives the DTMF signal-generating circuit in response to the operation of dial key DK, and causes it to produce a DTMF signal. The tone dialer produces a signal at a single frequency when receiving a predetermined test input signal. With this, the tone dialer produces a DTMF signal in response to the key data input from the microcomputer. Additionally, the tone dialer can check individual low and high group frequencies when newly-manufactured telephone sets are inspected. The output signal from the tone dialer, i.e. the single-frequency signal, when the tone dialer receives the test input, or the DTMF signal generated by the normal dial key operation, is output to the telephone network. This signal is also supplied to the receiver of the telephone set, to produce the sound corresponding to each frequency.
In the multifunction telephone set, no key-in sound is produced when those keys other than the dial keys, that is, the function keys such as re-dial keys and abbreviated dial key are operated. Also no key-in sound is produced when the dial key is operated for dial number registration. Thus, in such a case, the user cannot know if the key-in was properly performed. To eliminate this problem, a frequency-dividing circuit is provided inside or outside the microcomputer. The signal obtained by frequency-dividing the clock signal by means of the frequency divider, is used to produce the key-in sound, even when the special key is operated. Provision of the frequency divider dedicated for key-in sound generation, however, inevitably leads to an increase in the cost and size of the telephone sets.